1. Industrial Field of the Invention
This invention relates to an apparatus for vacuum deposition of a sublimative substance such as SiO.sub.2 or SiO provided with a device for heating the sublimative substance.
2. Description of the Prior Art
Heretofore, the formation of a thin film by the vacuum deposition of such a sublimative substance as SiO.sub.2 or SiO on base material, such as paper or plastic film, has been carried out with an apparatus constructed as illustrated in FIG. 3.
With reference to FIG. 3, base material 1 in the form of a roll is set on a takeup reel 6 inside a vacuum deposition chamber 2 as connected to a takeup reel 5 via a deflector roll 3 and a cooling roll 4. After the interior of the vacuum deposition chamber 2 is evacuated by an exhaust pump unit 12 to a prescribed degree of vacuum, the base plate 1 is fed by the takeup reel 6, subjected to vacuum deposition effected by a vacuum deposition device 7 while being cooled on the cooling roll 4 so as to decrease the temperature elevated as the result of the application of heat during the vacuum deposition, and is rewound on the takeup reel 5.
The vacuum deposition device 7, as illustrated in FIG. 4, evaporates vacuum deposition material 8. The device 7 includes a plurality of receptacles 9 arranged in the direction of width of the base material 1 as spaced at given intervals, and a heating device 10 for heating the receptacles 9. The vapor of the vacuum deposition material 8 flows toward the moving base material 1.
When the vacuum deposition is effected with aluminum, for example, the receptacles 9 in the art are made of graphite. When the vacuum deposition is performed with such a sublimative substance as SiO.sub.2 or SiO, however, no ideal material has yet been found for the receptacles 9.
During this vacuum deposition, the vacuum deposition material 8 which has been vaporized is sent flying not only directly upwardly but also obliquely outwardly from the vertical. To keep the vaporized vacuum deposition material from landing on the cooling roll 4 which is separated from the base material 1, an edge mask 11 forms a gap with each of opposite sides of the base material 1 and overlies opposite sides of the cooling roll.
Since the conventional device has the plurality of receptacles 9 spaced apart in the direction of width of the moving base material 1 as illustrated in FIG. 4, the thickness of the vacuum deposited film varies in the direction of width of the base plate 1. The vaporization may be caused by a reaction within the receptacles 9. For example, vacuum deposition materials such as SiO.sub.2 and Si are placed in the receptacles and are caused to undergo the following reaction at an elevated temperature to produce SiO vapor. EQU SiO.sub.2 +Si.fwdarw.2SiO (vapor)
Variations in the thickness of the vacuum deposited film occur in the direction of width depending on the positions of the receptacles 9 which establish the locations at which the above reaction occurs.
Incidentally, the speed of vaporization of the vacuum deposition material increases in accordance with increases in the temperature of the vacuum deposition material. When the vacuum deposition material is a fusible substance, the conventional heating device suffices because the vacuum deposition material is fused within the receptacles and is brought to a substantially uniform temperature therein. However, when the vacuum deposition material is a sublimative substance, since it is not fused but forms voids in the receptacle, the voids impair conduction of heat through the material and lower the apparent thermal conductivity thereof. The interior of the vacuum deposition material is characterized by temperature differences proportionate to the apparent thermal conductivity of the material. When part of the material, other than at the surface, is heated in the conventional device, the temperature of the receptacles is elevated to a point where problems occur. That is, the receptacles wear owing to the reaction between the receptacles and the vacuum deposition material.